High vinyl polybutadiene is a rubber which is utilized in a variety of applications. For example, high vinyl polybutadiene is sometimes used in preparing tire tread rubbers. This is because its utilization in tire treads provides good traction and rolling resistance. The high vinyl polybutadiene utilized in such applications typically contains at least about 60% repeat units which have the 1,2-microstructure.
High vinyl polybutadiene is commonly prepared utilizing lithium catalyst systems which are modified with a polar modifier. The polar modifier increases the vinyl content of the polybutadiene being synthesized. Ethers and tertiary amines which act as Lewis bases are commonly utilized as polar modifiers in conjunction with lithium catalyst systems. For example, U.S. Pat. No. 4,022,959 indicates that diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,N',N'-tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine, and N-phenyl morpholine can be used as modifiers. The vinyl group content of polydienes prepared utilizing such Lewis bases as modifiers depends upon the type and amount of Lewis base employed as well as the polymerization temperature utilized. For example, if a higher polymerization temperature is employed, a polymer with a lower vinyl group content is obtained (see A. W. Langer, A. Chem. Soc. Div. Polymer Chem. Reprints: Vol. 7 (1), 132 [1966]). For this reason, it is difficult to synthesize polymers having high vinyl contents at high polymerization temperatures utilizing typical Lewis base modifiers.
Higher temperatures generally promote a faster rate of polymerization. Accordingly, it is desirable to utilize moderately high temperatures in commercial polymerizations in order to maximize throughputs. U.S. Pat. No. 4,696,986 discloses that 1,2,3-trialkoxybenzenes and 1,2,4-trialkoxybenzenes can be utilized as modifiers for lithium catalyst systems to promote the formation of high vinyl polymers at temperatures which are high enough to promote fast polymerization rates.
Molybdenum catalyst systems have been utilized in the preparation of high vinyl polybutadiene. However, it has traditionally been necessary to utilize aromatic solvent systems in conjunction with such catalyst systems in order to attain acceptable conversions. The undesirable nature of utilizing such aromatic solvent systems has essentially precluded the utilization of molybdenum catalyst systems in commercial operations. Conventional molybdenum catalyst systems also lead to the production of high vinyl polybutadiene having extremely high molecular weights. For example, the utilization of such molybdenum catalyst systems typically leads to the production of high vinyl polybutadiene having a number average molecular weight of about 500,000. In most applications it is undesirable for the high vinyl polybutadiene to have a molecular weight which is that high. Accordingly, the high molecular weight of the polymer produced has also hindered the commercial development of molybdenum catalyst systems for preparing high vinyl polybutadiene.